An Amazon spokesperson said customers are eligible for the $20 credit if their order was fulfilled in time but couldn't be delivered by Christmas day. The credit should automatically appear on your account. Amazon will also refund shipping charges for the delayed orders.

So why were there so many delays?

UPS said it received far more orders this week than it had anticipated, so it couldn't deliver everything in time for Christmas. Deliveries should be back to normal today.

The delays caused a lot of customers to complain on Twitter using the #UPSFail hashtag.

Disclosure: Jeff Bezos is an investor in Business Insider through his personal investment company Bezos Expeditions.